Hafnium nitride film resistor

ABSTRACT

HAFNIUM NITRIDE THIN FILM RESISTORS ARE OBTAINED BY SPUTTERING HAFNIUM IN THE PRESENCE OF NITROGEN MAINTAINED AT PARTIAL PRESSURES WITHIN THE RANGE OF 10**4-10**2 TORR.

April 20, 1&7! GERSTENBERG El'AL 3,575,833

HAFNIUM NITRIDE FiLM RESISTOR Filed Feb. 26. 1968 FIG. 3

FIG. 2 /200- /000-- K? E 000+ s E 600-- k 400- -200 I I11 1 |11|m| 50PART/AL NITROGEN PRESSURE /N TOR/Q United States Patent p.s. fc1.204-192 2 Claims ABSTRACT OF THE DISCLOSURE I-Iafnium nitride thin filmresistors are obtained by sputtering hafnium in the presence of nitrogenmaintained at partial pressures within the range of -10" torr.

This invention relates to film resistors including a layer of hafniumnitride. More particularly, the present invention relates to filmresistors of high stability including a reactively sputtered layer ofhafnium nitride.

- Recently, considerable interest in the microminiaturization field hasbeen focused upon tantalum nitride film resistors of high stabilitywhich have been found to manifest electrical properties which comparefavorably with those resistors commonly employed by the prior artworkers. These structures are commonly prepared by reactively sputteringtantalum in the presence of nitrogen upon a suitabIe non-conductingsubstrate member. Although devices of this type have increased inpopularity lately, inherent limitations on the range of availableelectrical properties havev motivated a search for alternate structures.

In accordance with the present invention, the limitations alluded tohereinabove are effectively obviated by reactively sputtering hafniumupon a suitable substrate member in the presence of nitrogen at nitrogenpartial pressures ranging from l0*1O* torr. Devices described hereinhave been found to evidence a wider range of homogeneity and,consequently, a wider range of electrical properties such as temperaturecoefficient of resistance and resistivity than the tantalum nitridestructures; Specifically, it has been found that the resistorsfabricated in accordance with the invention evidence resistivitiesranging from 150-1000 microhm-centimeters over a range of temperaturecoefiicients of resistance ranging from +500 to 500 parts per millionper degree centigrade.

FIG. 2 is a graphical representation on coordinates of-resistivity inmircrohm-centimeters and temperature coefiicient of resistance in partsper million per degree centigrade' against the partial pressure ofnitrogen in torr. showing the variations of resistivity and temperaturecoefi'icient of resistance in varying partial pressures of nitrogen at atotal pressure of 20 millitorr. of argon; and

FIG. 3 is a plan view of a hafnium nitride film resistor prepared inaccordance with the present invention. 3 With reference now moreparticularly to FIG. 1, there is shown an apparatus suitable fordepositing a hafnium nitride film by cathodic sputtering. Shown in thefigure 3,575,833 Patented Apr. 20, 1971 is a vacuum chamber 11 in whichare disposed cathode 12 and anode 13. Cathode 12 may be composed ofhafnium or, alternatively, may serve as the base of the hafnium whichlater may be in the form of a coating, foil, or other suitable physicalform.

A source of electrical potential 14 is shown connected between cathode12 and anode 13. Platform 15 is employed as a positioning support forsubstrate 16 upon which the sputtered film is to be deposited. Mask 17is placed upon substrate 16 to restrict the deposition to this area.

The present invention is conveniently described in detail by referenceto an illustrative example in which hafnium is employed as cathode 12 inthe apparatus shown in FIG. 1. Preferred substrate materials for thepractice of the present invention are glasses, glazed ceramics, and soforth. These materials meet the requirements of heat resistance andnon-conductivity essential for substrates utilized in reactivesputtering techniques.

Substrate 16 is first vigorously cleaned. Conventional cleaning agentsare suitable, the choice of a specific one being dependent upon thecomposition of the substrate itself. For example, when the substrateconsists of glass, boiling in aqua regia or hydrogen peroxide is aconvenient method for cleaning the surface.

Substrate 16 is placed upon platform 15, as shown in FIG. 1 and mask 17is then suitably positioned. Platform 15 and mask 17 may be fabricatedfrom any refractory material. However, it may be convenient to use ametal such as aluminum for ease in the fabrication of mask 17. In orderto obtain sharply defined paths, it is necessary to have mask 17 bearingupon substrate 16 under externally applied pressure.

The vacuum chamber is next evacuated and nitrogen is admitted at adynamic pressure, and after obtaining equilibrium, argon is admitted.The extent of the vacuum is dependent upon consideration of severalfactors.

Increasing the inert gas pressure and thereby reducing the vacuum withinchamber 11 increases the rate at which the hafnium being sputtered isremoved from the cathode and, accordingly, increases the rate ofdeposition. The maximum pressure is usually dictated by power supplylimitations since increasing the pressure also increases the currentflow between cathode 13 and anode 12. A practical upper limit in thisrespect is 20 millitorr. for a sputtering voltage of 3000 volts,although it may be varied depending on the size of the cathodesputtering rate and so forth. The ultimate maximum pressure is that atwhich the sputtering can be reasonably controlled within the prescribedtolerances. It follows from the above discussion that the minimumpressure is determined by the lowest deposition rate which can beeconomically tolerated.

After the requisite pressure is obtained, cathode 12 which may becomposed by hafnium or, alternatively, may be an aluminum disk coveredwith hafnium, for example, in the form of a foil, is made electricallynegative with respect to anode 13.

The minimum voltage necessary to produce sputtering is about 3000 volts.Increasing the potential difference between anode 13 and cathode 12 hasthe same effect as increasing the pressure, that of increasing both therate of deposition and the current flow. Accordingly, the maximumvoltage is dictated by consideraiton of the same factors controlling themaximum pressure.

The spacing between anode and cathode is not critical. However, theminimum separation is that required to produce a glow discharge whichmust be present for sputtering to occur. Many dark striations are wellknown and have been given names, as, for example, Crookes Dark Space(see 1003 Theoretical PhysicsHafner, New York, 1950, page 435 etc.). Forthe best efficiency during the sputtering step, substrate 16 should bepositioned immediately without the Crookes Dark Space on the sideclosest to the anode 13. Location of substrate 16 closer to cathode 12results in a metal deposit of poorer quality. Locating substrate 16further from cathode 12 results in the impingement on the substrate by asmaller fraction of the total metal sputtered, thereby increasing thetime necessary to produce a deposit of given thickness.

It should be noted that the location of Crookes Dark Space changes withvariations in the pressure, it moving closer to the cathode withincreasing pressure. As the substrate is moved closer to the cathode ittends to act as an obstacle in the path of gas ions which are bombardingthe cathode. Accordingly, the pressure should be maintained sufficientlylow so that Crooks Dark Space is located upon the point at which asubstrate would cause shielding of the cathode.

The balancing of these various factors are voltage, pressure, andrelative position of the cathode, anode, and substrate to obtain a highquality deposit is well known in the sputtering art.

With reference now more particularly to the example under disucssion, byemploying a proper voltage, pressure, and spacing of the variouselements within the vacuum chamber, a layer of hafnium nitride isdeposited in a configuration determined by mask 17. The sputtering isconducted for a period of time calculated to produce the desiredthickness.

For the purposes of this invention, the mninmum thickness of the layerdeposited upon the substrate is approximately 400 A. There is no maximumlimit on this thickness although little advantage is gained by anincrease upon 1500 A.

FIG. 2 is a graphical representation showing the resistivity inmicrohm-centimeters and temperature coefficient of resistance in partsper million per degree centigrade against the partial pressure ofnitrogen in torr. The points on the graph represent the average over theinmer six resistor strips on nichrome-gold terminated 1 x 3" glassslides which have been sputtered at a temperature of 350 C. at 4.0kilovolts at a current density of 0.43 milliampere per squarecentimeter.

It is noted from the graph that at partial nitrogen pressuresappreciably below 10* torr. there is little change in resistivityaccompanied by a large increase in the temperature coefiicient ofresistance from +700 parts per million per degree centigrade to +1200parts per million per degree centigrade. Above a partial pressure ofnitrogen of torr., the resistivity increases from 100microhm-centimeters to a peak of 300 microhmcentimeters, correspondingwith a partial pressure of 10* torr. Thereafter, resistivity decreasesto a low point of approximately 100 microhm-centimeters and rapidlyincreases to values greater than 1200 microhm-centimeters at partialpressures of the order of 10' torr. It has been found that thetemperature coefficient of resistance fluctuates from approximately +500parts per million per degree centigrade to -500 parts per million perdegree centigrade over a partial nitrogen pressure range of 10 40- Inanalyzing the data shown graphically in the figure, it must be notedthat the indicated pressures are specific to the pumping speed of theparticular vacuum system employed. Thus, it may be stated that thepresent invention is operable over a partial pressure of nitrogen rangeof 10- --10- torr.

In FIG. 3 there is shown a substrate member 21 composed of one of therefractory insulating materials usually employed in the construction ofprinted circuit boards which has deposited thereon two terminals, 21aand 21b, of eletcrically conductive metal such as gold and a layer 23 ofhafnium nitride. Conductive terminals 21a and 21b are not essential butare customarily employed in the construction of printed circuit boards.With reference once again to the example under discussion, the substrateis maintained at temperatures within the range of 300-500 C. during thereactive sputtering process. Following the deposition technique, thehafnium nitride film is heated to a temperature within the range of250-400 C. in the presence of air, thereby stabilizing the nitride film.Electron diffraction studies indicate that reactively sputtered hafniumnitride films have porperties suitable for resistor purposes and, whenproduced in accordance with the techniques described herein, are of aface-centered cubic structure (Hf-N).

Several examples of the present invention are described in detail below.These examples and the illustration described above are included merelyto aid in the understanding of the invention and variations may be madeby one skilled in the art without departing from the spirit and scope ofthe invention.

EXAMPLE I This example describes the fabrication of a hafnium nitrideresistor in accordance wtih the persent inventive technique.

A sputtering apparatus similar to that shown in FIG. 1 was used toproduce the hafnium nitride layer. The cathode consisted of a circularhafnium disk 10 centimeters in diameter of high purity. In the apparatusactually employed, the anode was grounded, the potential differencebeing obtained by making the cathode negative with respect to ground.

A glass microscope slide, approximately 1" in width and 3" in length wasused as a substrate. Nichrome gold terminals were evaporated on eachlongitudinal side of the substrate. The terminated slide was thencleaned using the following procedure. Initially, the slide was washedin a detergent to remove large particles of dirt and grease. Next, therefollowed a tap water rinse, a 10 minute boil in a 10 percent hydrogenperoxide solution, a distilled water rinse, a 10 minutes boil indistilled water, and storage in an oven maintained at C. until ready foruse.

The vacuum chamber was evacuated by means of an oil diffusion pump to apressure of approximately 5X10", torr. after a time period within therange of 15 to 45 minutes. Next, the substrate was heated to atemperature of approximately 400 C. at which point nitrogen was admittedinto the chamber at a dynamic pressure, and, after attainingequilibrium, argon was admitted into the chamber at a pressure ofapproximately 20 10- torr. During the sputtering reaction, the partialpressure of the nitrogen was maintained at approximately 2 1Cttorr.

The anode and cathode were spaced approximately 2 /2 apart, .thecleansed substrate being placed therebetween at a position immediatelywith the Crookes Dark Space. The substrate was maintained at atemperature of 350 C. during the sputtering reaction. A DC voltage of4000 volts was impressed between cathode and anode at a current densityof 0.43 milliampere per square centimeter. In order to establishequilibrium when first beginning the sputtering operation, it was foundhelpful to sputter on a shield for several minutes thereby issuingreproducible results. Sputtering was conducted for approximately 8minutes, resulting in a layer of approximately 1100 A. in thickness.

Following the sputtering treatment, the resistance in ohms and specificresistivity in microhm-centimeters was measured. Next, the sputteredresistor was heated in air for 1 hour at a temperature of 425 C. Thedevice so fabricated was again measured to determine its resistance.Stability of the resistor was determined by power aging at 1 volt forapproximately 1000 hours. The results are set forth in the tabel belowtogether with the results obtained by repetition of the foregoingprocedure. Each of the results set forth in the table is to beconsidered as an independent example.

TABLE Initial Res. in AB 90 (425 C.) afterin A. in torr. ohms heattreat. 24 hrs. 108 hrs. 604 hrs. 1,008 hrs.

(1) 1, 100 2X10 2, 705 2, 789 0. 118 0. 191 0. 225 0. 228 (2).--- 1, 1002X10 2, 442 2, 518 0. 106 0. 182 0. 218 0. 265 (3) 1, 100 2X10' 2, 4722, 549 0. 122 0. 193 0. 239 0. 290 (4) 1, 100 2X10- 2, 576 2, 656 0. 1130. 181 0. 225 0. 270 (5) 1, 100 2X 3, 519 3, 628 0. 102 0. 153 0. 168 0.230

F.'I.=Film thickness.

PN2=Partial pressure of nitrogen.

Analysis of the data set forth in the table indicates that theresistance of the hafnium nitride films prepared in accordance with theinvention are appreciably enhanced by the heat treatment, so producing auniquely stable resistor as noted from the aging data. Furthermore, asmay be seen by reference to FIG. 2, the hafnium nitride films manifestresistivities ranging from 150-1000 microhmcentimeters over the nitrogenpartial pressure range of interest herein, whereas the temperaturecoefficient of resistance over this range varies from +500 to 500 partsper million per degree centigrade. This behavior is superior to that oftantalum nitride which shows only modest changes in resistivity andtemperature coefiicient of resistance over the nitrogen partial pressureranges employed in the fabrication of such structures.

While the invention has been described in detail in the foregoingspecification, and the drawing similarly illustrates the same, theaforesaid is by way of illustration only and is not restrictive incharacter. It will be appreciated by those skilled in the art that thenovel resistors may be fabricated by methods other than reactivesputtering; such methods being well known by those skilled in the art.The several modifications which will readily suggest themselves topersons skilled in the art are all considered within the scope of theinvention, reference being had to the appended claims.

References Cited UNITED STATES PATENTS 3,242,006 3/1966 Gerstenberg117-201 3,233,988 2/1966 Wentorf et a1. 23-191 3,170,810 2/ 1965 Kagan204-192 FOREIGN PATENTS 603,282 8/1960 Canada 23-191 TA-HSUNG TUNG,Primary Examiner S. S. KANTER, Assistant Examiner US. Cl. X.R.

